Fluorine-containing resin composition, method for producing the same, and cable having coating comprising the same

ABSTRACT

The present invention provides a fluororesin composition, which is excellent in moldability and crack resistance and particularly, does not cause cone break when coating an electric wire, a process for preparing the composition and a cable coated with the composition. Specifically, the present invention provides a fluororesin composition comprising a tetrafluoroethylene/hexafluoropropylene copolymer and polytetrafluoroethylene compounded thereto, which has a melt flow rate of at least 15 at 372° C. Also, the present invention provides a process for preparing the composition which comprises kneading with a multi-screw kneader. Furthermore, the present invention provides a cable which is coated with the resin.

TECHNICAL FIELD

[0001] The present invention relates to a fluororesin composition, whichis excellent in moldability and in which a melt processabletetrafluoroethylene/hexafluoropropylene copolymer andpolytetrafluoroethylene are compounded, a process for preparing the sameand a cable coated with the same.

BACKGROUND ART

[0002] A copolymer of tetrafluoroethylene (hereinafter referred to asTFE) and hexafluoropropylene (hereinafter referred to as HFP)(hereinafter referred to as FEP) has heat resistance, chemicalresistance, electric insulating properties, non-adhesiveness and lowfrictional properties equal to those of polytetrafluoroethylene(hereinafter referred to as PTFE) and therefore is often applied notonly in the chemical industry, electric and electronic industries andmachine industry, but also in a wide range of fields, from spacedevelopment and aircraft industry to household products. Furthermore,FEP has melt flowability and can be melt processed by compressionmolding, extrusion molding, injection molding and fluid immersioncoating.

[0003] However, in comparison to general thermoplastic resin, FEP has anextremely high melt flow temperature and high melt viscosity andtherefore, a molding temperature of 330° to 420° C., in some cases 440°C., is required. Molding at such high temperatures causes thermaldecomposition of the polymer. Furthermore, when extrusion pressure israised in order to increase the molding speed, surface roughness of theextruded article and abnormal flow (melt fracture) are caused.

[0004] In order to solve the above problems and improve moldabilitywithout losing properties, JP-A-52-98761 discloses a fluororesincomposition, in which PTFE is compounded in FEP. As a result, phenomenasuch as surface roughness of the molded article and melt fracture aresaid to have difficulty occurring, but the effects are stillinsufficient.

[0005] Also, in recent years, FEP is used for cable coating material andthe speed for molding may reach at least 1200 ft/minute and in somecases, 3000 ft/minute. This speed is within the melt fracture range ofFEP. Consequently, the coated cable is large in wire diameter unevennessand electric properties such as insulation become uneven. Furthermore,the phenomenon of cone break occurs, that is the conular part formed inthe die when coating comes off, and coating at high speed is difficult.

[0006] Furthermore, in order to improve electric properties, an electricwire can be coated with resin in a foamed state. However, to obtainstable electric properties, the resin must be in a foamed state of evenbubbles with a small bubble diameter.

DISCLOSURE OF INVENTION

[0007] The present invention solves the above problems and provides afluororesin composition, which is excellent in moldability and crackresistance and particularly, does not cause cone break when coating anelectric wire, a process for preparing the composition and a cablecoated with the composition.

[0008] That is, the present invention relates to a fluororesincomposition comprising a tetrafluoroethylene/hexafluoropropylenecopolymer and polytetrafluoroethylene compounded thereto; thecomposition having a melt flow rate of at least 15 at 372° C.

[0009] The content of the polytetrafluoroethylene is preferably 0.03 to2 parts by weight based on 100 parts of the copolymer.

[0010] The present invention also relates to a process for preparing afluororesin composition which comprises kneading atetrafluoroethylene/hexafluoropropylene copolymer andpolytetrafluoroethylene with a multi-screw kneader.

[0011] The multi-screw kneader is preferably a twin-screw extruder.

[0012] The multi-screw kneader preferably has at least one kneadingblock.

[0013] Kneading is preferably conducted while adding water.

[0014] Kneading is preferably conducted while adding a gas containingoxygen.

[0015] The copolymer and polytetrafluoroethylene are preferablypre-mixed.

[0016] The present invention also relates to a cable comprising a corewire coated with the fluororesin composition.

[0017] The core wire of the cable preferably comprises metal.

[0018] The metal is preferably copper.

[0019] The present invention also relates to a cable comprising a corewire coated with the fluororesin composition, which is foamed by inertgas.

[0020] The inert gas is preferably carbon dioxide or nitrogen.

[0021] The core wire of the cable preferably comprises metal.

[0022] The metal is preferably copper.

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] The copolymer used in the present invention comprises TFE and HFPand the composition ratio thereof is preferably 75 to 90% by weight ofTFE and 10 to 25% by weight of HFP based on the copolymer weight. Morepreferably, TFE is 80 to 90% by weight and HFP is 10 to 20% by weight.When TFE is less than 75% by weight, moldability and heat resistancetend to become poor as the melting point becomes low. When TFE is morethan 90% by weight, the melting point becomes high and moldability tendsto become poor. When HFP is less than 10% by weight, crack resistancetends to become poor. When HFP is more than 25% by weight, moldabilitybecomes poor as the melting point becomes low and heat resistance andproductivity tend to become poor. Also, the polymerization speed tendsto become slow. The method for copolymerization is not particularlylimited and examples are suspension polymerization and emulsionpolymerization. The copolymer may also contain a small amount of a thirdcomponent, a modifying agent or a filler.

[0024] As the third component added to the copolymer, perfluoroalkylvinylether is preferable. By adding perfluoroalkyl vinylether as acopolymer component, moldability and crack resistance tend to improvefurther. Examples of the perfluoroalkyl vinylether are vinyletherrepresented by the formula:

CF₂═CFO (CF₂)_(m)F

[0025] (wherein m is an integer of 1 to 6)(perfluoromethyl vinylether(hereinafter referred to as PMVE) when m is 1) and vinyletherrepresented by the formula:

CF₂═CF—(O—CF₂CF(CF₃))_(n)OC₃F₇

[0026] (wherein n is an integer of 1 to 4). When n is 1 to 2, heatresistance is poor as the melting point becomes low and when n is 4,production costs tend to increase. Therefore, perfluoropropyl vinyletherin which n=3 (hereinafter referred to as C3VE) is preferable. Also, thecopolymer composition ratio is preferably 67 to 89.5% by weight of TFE,10 to 20% by weight of HFP and 0.5 to 13% by weight of perfluoroalkylvinylether based on the polymer of TFE/HEP/perfluoroalkyl vinylether(hereinafter referred to as ternary copolymer). More preferably, TFE is83 to 89.3% by weight, HFP is 10 to 15% by weight and perfluoroalkylvinylether is 0.7 to 2% by weight. When TFE is less than 67% by weight,heat resistance tends to become extremely poor and when TFE is more than89.5% by weight, moldability is poor as the melting point becomes highand crack resistance tends to become poor. When HFP is less than 10% byweight, crack resistance tends to become poor and when HFP is more than20% by weight, moldability becomes poor as the melting point becomes lowand heat resistance and productivity tend to become poor. Whenperfluoroalkyl vinylether is less than 0.5% by weight, the effect ofcopolymerizing perfluoroalkyl vinylether tends not to appear. Whenperfluoroalkyl vinylether is more than 13% by weight, the viscositybecomes low and moldability tends to become poor. In this case, the meltflow rate at 372° C. is at least 15. The ternary copolymer may alsocontain a small amount of a fourth component, a modifying agent or afiller. The method for copolymerization is not particularly limited andexamples are suspension polymerization and emulsion polymerization.

[0027] The content of PTFE compounded with the copolymer is preferably0.03 to 2 parts by weight based on 100 parts by weight of the copolymer.More preferably, the lower limit is 0.05 part by weight and the upperlimit is 0.5 part by weight. When PTFE is less than 0.03 part by weight,the effect of adding PTFE tends not to appear. When the amount is morethan 2 parts by weight, the melt viscosity increases significantly andthe molded article tends to become brittle. PTFE preferably has amolecular weight of at least 1,000,000, more preferably at least3,000,000. When the molecular weight is less than 1,000,000, the moldedarticle tends to be poor in crack resistance. PTFE is a homopolymer ofTFE or a copolymer of TFE and hexafluoropropylene, tetrafluoroethyleneor tetrafluoro ethylene chloride. In the case that PTFE is compounded aspowder, the particle size is preferably at most 1,000 μm, morepreferably at most 800 μm. When the particle size of PTFE is more than1,000 μm, dispersability with the copolymer tends to be inhomogeneous.The form of PTFE is not limited and can be powder, a dispersion or asuspension.

[0028] The fluororesin composition of the present invention is preparedby mixing the above copolymer and PTFE and has a melt flow rate(hereinafter referred to as MFR) measured according to ASTM D-2116 of atleast 15 at 372° C. The upper limit is preferably 45 and morepreferably, the lower limit is 17.5 and the upper limit is 42.5. Whenthe MFR at 372° C. is less than 15, the flowability is low and problemsoccur when using as, for example, coating material for a cable. That is,as mentioned above, the speed for coating a cable is extremely fast andtherefore, when the MFR at 372° C. is less than 15, the conular part(cone part) formed in the die when coating comes off and the cablecannot be coated. When the MFR is more than 45, maintaining the conepart when coating is also difficult and coating of an electric wiretends to become difficult.

[0029] The copolymer and PTFE used in the present invention are kneadedby a multi-screw kneader. It is important that the PTFE is sufficientlydispersed. When dispersion is insufficient, PTFE with high molecularweight aggregates when molding. For example, when a mixture of thecopolymer and PTFE containing PTFE aggregate is used as coating materialfor a cable, the surface of the cable becomes uneven due to the PTFEaggregate, adversely affecting electric properties. However, asmentioned above, because the amount of PTFE is an extremely small amountof preferably 0.03 to 2 parts by weight, homogeneously dispersing isdifficult. Therefore, a twin-screw extruder, which is particularlyexcellent in kneading function, is preferably used. More preferably, amulti-screw kneader having at least one kneading block is used. Thenumber of kneading blocks can be determined according to the functionsof the kneader. The kneading block can further improve the degree ofdispersion of PTFE. Preferably, from the viewpoint of advancingkneading, as many kneading blocks as possible are installed within therange that the copolymer and PTFE are not damaged.

[0030] Kneading by the multi-screw kneader is preferably conducted whileadding water or air. Usually, water and air introduced into the extruderare removed by forming a vent hole in a cylinder and connecting a vacuumpump to the vent hole. However, by adding water or oxygen to the resin,the unstable terminal groups present in the fluororesin composition ofthe present invention can be stabilized. The unstable terminal groupsare thought to cause air bubbles, voids and coloring in the moldedarticle of the fluororesin composition.

[0031] Water stabilizes carboxylic acid terminal groups and convertsacid fluoride into carboxylic acid. The amount of water added must be anamount excessive of the theoretical amount required for the reaction.The amount is largely influenced by the pressure when adding and thestate within the extruder and is determined while checking with anactual machine. The reaction for stabilizing carboxylic acid terminalgroups is significantly advanced by adding a reaction accelerating agentsuch as an alkali metal compound and so water is preferably added as anaqueous solution in which an accelerating agent is dissolved. Theconcentration of the aqueous solution can be determined accordinglybased on the necessary amount of the accelerating agent. As long aswater is present in the kneader within the area in which stabilizationtreatment is conducted (hereinafter referred to as stabilizationtreatment area), the fluorine-containing polymer may be moistened tocontain water before being added into the kneader or water may besupplied after adding the dried polymer into the kneader. Herein, thestabilization treatment area can be located, for example, at the screwpart immediately after the melting zone formed by the kneading block ofthe twin-screw extruder. Also, modifications are possible, such asforming a long melting zone and the stabilization treatment area furtherdownstream. Stabilization can theoretically be conducted if the amountof water supplied has the same number of molecules as the number ofunstable terminal groups which develop in the kneader. However, inreality, an excessive amount of water, particularly water with at least10 times the number of molecules of the number of unstable terminalgroups, is preferably supplied. The upper limit is not particularlydefined.

[0032] Oxygen converts vinyl terminal groups into acid fluoride terminalgroups. Furthermore, the function of oxidizing the few carbon atomsproduced in depolymerization of the vinyl terminal groups to producecarbon dioxide can be obtained.

[0033] The amount of oxygen (O₂) present differs depending on thetemperature at the reaction, the residence time in the stabilizationtreatment area, the type of extruder and the type and amount of unstableterminal groups. The amount is at least an amount equimolar to theamount of unstable terminal groups (—CF═CF₂), which need to bestabilized. When considering diffusion loss and the amount dischargedwithout contributing to the reaction, the amount is preferably an excessamount, for example at least 10 times the molar amount, particularly 50to 500 times the molar amount.

[0034] As the gas containing oxygen, oxygen can be diluted to a suitableconcentration (for example 10 to 30% by volume) with inert gas such asnitrogen gas and argon gas and then supplied but preferably air is usedas it is, from an economical viewpoint.

[0035] As long as oxygen is present in the stabilization treatment area,oxygen may be contained in the fluorine-containing polymer before beingadded into the kneader or oxygen may be supplied after thefluorine-containing polymer is added into the kneader. Naturally, bothmethods may be used together.

[0036] Also, kneading while simultaneously supplying water and oxygen(air) is preferable.

[0037] The treatment time, that is the residence time, in thestabilization treatment area differs depending on the structure of thekneader, the supply method of water and air and the treatmenttemperature in the stabilization treatment area. Usually, less than 10minutes is sufficient and 0.2 to 5 minutes is preferable. When theresidence time becomes long, large shearing force is applied and thepolymer tends to become damaged.

[0038] The temperature of the stabilization treatment area is usually200 to 450° C., preferably 300 to 400° C.

[0039] In the present invention, gaseous substances produced in thestabilization treatment reaction, such as hydrogen fluoride, carbondioxide and small amounts of monomers produced by decomposition, aredrawn out from within the stabilized fluorine-containing polymer anddischarged from the kneader. Therefore, a degassing area, in which theabsolute pressure is maintained at 0.1 MPa or lower, is preferablylocated in the kneader successively to the stabilization treatment area.The absolute pressure in the degassing area differs depending on themelted state of the polymer and operation conditions such as therotational speed of the extruder screw. The pressure is preferablyreduced to a degree at which the polymer does not enter into thedischarge nozzle.

[0040] The fluorine-containing polymer obtained by stabilizationtreatment and discharged from the kneader of the present inventionusually has the shape of a pellet. When the pellet is used in meltmolding, air bubbles and voids do not develop and coloring does notoccur in the obtained molded article.

[0041] When necessary, the kneaded object (pellet) retrieved from thekneader can be subjected to the above fluorination treatment.

[0042] Before kneading in a multi-screw extruder, the copolymer and PTFEare preferably pre-mixed, thereby further improving the degree ofdispersion of PTFE. The powder mixer used for pre-mixing is notparticularly limited and examples are a mixer, a mixing roll, a kneader,a ball mill, a banbury mixer and a blender. The pre-mixing method can bea dry method or a wet method. Furthermore, the copolymer and PTFE can beadded to the reaction system before or during the polymerizationreaction of the copolymer to be pre-mixed.

[0043] The cable of the present invention is coated with the fluororesincomposition of the present invention, in which the copolymer and PTFEare compounded. Because the fluororesin composition has high flowabilityof MFR of at least 15 at 372° C., even when molded at high speed, conebreak does not occur and the wire diameter becomes even. As a result, acable which is excellent in electric properties such as insulation canbe obtained.

[0044] An example of the cable is a LAN cable used for constructing alocal area network (LAN) of personal computers. A LAN cable is usedinside an office and therefore is required to have high flameretardancy. The fluororesin composition of the present invention canfulfill this requirement. The thickness of the resin composition coatingof the cable can be determined according to the use.

[0045] The core wire of the cable is preferably metal. Particularly,copper is preferable.

[0046] Coating the cable with the fluororesin composition in a foamedstate can be achieved by adding inert gas into the extruder when moldingthe electric wire. Inert gas dissolves relatively well in the FEP part,which has amorphous parts, and becomes bubbles when discharged from thedie of the electric wire molding machine. The growth of the bubblesseems to stop due to the crystalline parts of the added PTFE andrelatively small bubbles can be produced. The inert gas is notparticularly limited as long as it is a gas which does not react withresin and from the viewpoints that availability and low cost, carbondioxide or nitrogen is preferable.

[0047] Hereinafter, specific Examples are described for explanation ofthe present invention, but the present invention is not limited thereto.

[0048] Measurement Method

[0049] (Lamp)

[0050] Detection was conducted of parts in which the resin coatingthickness was (preset diameter+20 mil) by a lamp detector (made byZumback, KW32 TRIO).

[0051] (Spark)

[0052] Detection was conducted of parts which were not coated with resinby a spark tester charged to 1.83 kV (made by CLINTON INSTRUMENTCOMPANY, Model HF-20-H).

[0053] (Cone break)

[0054] Cone break was considered to occur when both the lamp detectorand spark tester were simultaneously set off and continuing the test wastemporarily not possible as the cone part was visually found to havebroken.

[0055] (Measurement of Melt Flow Rate (MFR))

[0056] Melt flow rate was measured according to ASTM D-2116 by themethod of 372° C. and 5 kg load.

EXAMPLE 1

[0057] 100 parts by weight of FEP powder obtained by emulsionpolymerizing TFE/HFP/C3VE in an amount of 85.8/13.2/1.0 (weight ratio)with ammonium persulfate (APS) as a polymerization initiator and 0.15part by weight of PTFE fine powder (average particle size 450 μm, numberaverage molecular weight 5,000,000) were pre-mixed for 30 minutes by apowder mixer equipped with a stirrer and kneading block.

[0058] The above powder (containing air) to which special pre-dryingtreatment was not conducted except that potassium carbonate was added soas to become 20 ppm (value converted to amount of potassium, equal to4.3% of the total number of unstable terminal groups) was supplied at aspeed of 50 kg/hour into a twin-screw extruder made by Japan SteelWorks, Ltd. (screw shaft diameter: 47 mm, total length: 2468 mm, numberof barrels: 15 (first barrel: raw material supplying area, second tofifth barrels: melting area, sixth to twelfth barrels: kneading andreacting area, thirteenth to fifteenth barrels: degassing area)). At anarea further downstream from the powder supply port, deionized water andair (oxygen concentration approximately 20%) were supplied into thestabilization treatment area (kneading block area) at a flow rate of 1.5kg/hour and 40 NL/minute respectively. The preset temperature of thestabilization treatment area was 380 to 420° C. and the absolutepressure was 2.5 MPa and the total time required for the all treatmentincluding the time for melting by heating was 5 minutes (the residencetime in the stabilization treatment area was presumed to beapproximately 2 minutes). In this way, the fluororesin composition ofthe present invention in the form of pellets was obtained. The MFR ofthe fluororesin composition was 32.3 at 372° C.

[0059] The obtained pellets were coated on a copper wire with a diameterof 20.1 mil by a 2-inch screw extruder and the cable of the presentinvention with a diameter of 35.5 mil was obtained. The coating speedwas 1600 to 2800 ft/minute. In the extruder for coating an electricwire, the temperature was controlled in 5 zones in the main body and in4 zones in the die. From the raw material supply side, each temperaturewas controlled to

[0060] Main body: 327° C., 371° C., 388° C., 399° C., 416° C.

[0061] Die: 427° C., 427° C., 427° C., 427° C.

[0062] The coated cable was cooled by passing through an air-coolingzone and a water-cooling zone.

[0063] The results of the above measurement methods are shown in Table1.

COMPARATIVE EXAMPLE 1

[0064] The fluororesin composition (MFR at 372° C.: 32.3) was obtainedby the same method as in Example 1, except that PTFE fine powder was notused, and was coated on a copper wire.

[0065] The results of the above measurement methods are shown in Table1.

COMPARATIVE EXAMPLE 2

[0066] The fluororesin composition (MFR at 372° C.: 10) was obtained bythe same method as in Example 1, except that 100 parts by weight of FEPpowder obtained by emulsion polymerizing TFE/HFP in an amount of86.5/13.5 (weight ratio) with ammonium persulfate (APS) as apolymerization initiator and 0.15 part by weight of PTFE fine powder(average particle size 450 μm, number average molecular weight5,000,000) were pre-mixed for 30 minutes by a powder mixer equipped witha stirrer. Coating of a copper wire was attempted, but could not beconducted as cone break occurred even at a coating speed of 1600ft/minute. TABLE 1 Ex. 1 Com. Ex. 1 Lamp 1600 ft/minute 0 0(quantity/10^(×4) feet) 2000 ft/minute 1.6 8.3 2400 ft/minute 6.1 8.22800 ft/minute 1.3 11.9 Cone Break 1600 ft/minute 0 0 (times/10^(×4)feet) 2000 ft/minute 0 0 2400 ft/minute 0 8.2 2800 ft/minute 0 6 Spark1600 ft/minute 4.5 5.1 (quantity/10^(×4) feet) 2000 ft/minute 0 6.3 2400ft/minute 1.5 20.6 2800 ft/minute 1.3 14.9

Industrial Applicability

[0067] According to the present invention, a fluroresin composition canbe obtained, which is melt processable and excellent in moldability andcrack resistance and particularly, does not cause cone break whencoating an electric wire. Also, a cable evenly coated therewith can beobtained.

1. A fluororesin composition comprising atetrafluoroethylene/hexafluoropropylene copolymer andpolytetrafluoroethylene compounded thereto; said composition having amelt flow rate of at least 15 at 372° C.
 2. The fluororesin compositionof claim 1, wherein the content of said polytetrafluoroethylene is 0.03to 2 parts by weight based on 100 parts of said copolymer.
 3. A processfor preparing a fluororesin composition which comprises kneading atetrafluoroethylene/hexafluoropropylene copolymer andpolytetrafluoroethylene with a multi-screw kneader.
 4. The process forpreparing a fluororesin composition of claim 3, wherein said multi-screwkneader is a twin-screw extruder.
 5. The process for preparing afluororesin composition of claim 3, wherein said multi-screw kneader hasat least one kneading block.
 6. The process for preparing a fluororesincomposition of claim 3, wherein kneading is conducted while addingwater.
 7. The process for preparing a fluororesin composition of claim3, wherein kneading is conducted while adding a gas containing oxygen.8. The process for preparing a fluororesin composition of claim 3,wherein said copolymer and polytetrafluoroethylene are pre-mixed.
 9. Acable comprising a core wire coated with the fluororesin composition ofclaim
 1. 10. The cable of claim 9, wherein said core wire comprisesmetal.
 11. The cable of claim 10, wherein said metal is copper.
 12. Acable comprising a core wire coated with the fluororesin composition ofclaim 1, which is foamed by inert gas.
 13. The cable of claim 12,wherein said inert gas is carbon dioxide or nitrogen.
 14. The cable ofclaim 12, wherein said core wire comprises metal.
 15. The cable of claim14, wherein said metal is copper.